Methocarbamol, sold under the brand name Robaxin among others is a medication used for short-term musculoskeletal pain. It may be used together with rest and pain medication. It is less preferred in low back pain. It is not useful for cerebral palsy. Effects generally begin within half an hour. It is taken by mouth or injection into a vein.
|Trade names||Robaxin, others|
|By mouth, intravenous|
|Elimination half-life||1.14–1.24 hours|
|CompTox Dashboard (EPA)|
|Chemical and physical data|
|Molar mass||241.241 g/mol g·mol−1|
|3D model (JSmol)|
|(what is this?)|
Common side effect include sleepiness and dizziness. Serious side effects may include anaphylaxis, confusion, liver problems, and seizures. Use is not recommended in pregnancy and breastfeeding. Methocarbamol is a centrally acting muscle relaxant. How it works is unclear, but it does not appear to affect muscles directly.
Methocarbamol was approved for medical use in the United States in 1957. It is available as a generic medication. A month supply in the United Kingdom costs the NHS about 12.72 £ as of 2019. In the United States the wholesale cost of this amount is about US$6.50. In 2016 it was the 187th most prescribed medication in the United States with more than 3 million prescriptions.
Potential side-effects include: drowsiness, dizziness, clumsiness (ataxia), upset stomach, flushing, blurred vision, and fever. Both tachycardia (fast heart rate) and bradycardia (slow heart rate) have been reported; these can be serious. Other serious side-effects include the development of a severe skin rash or itching, fainting, jaundice, persistent nausea/vomiting, stomach/abdominal pain, mental/mood changes, trouble urinating, and signs of infection. If taken in large amounts at once or more than directed or as prescribed, dysphoria or suicidal thoughts may occur. Methocarbamol may cause urine to turn black, blue or green but the effect is harmless.
Methocarbamol has a high therapeutic index, i.e., a wide range of safe and effective dosages. Consumer (OTC) doses are in the range 3–6 g per day, while clinical doses can be as high as 24 g per day for severe conditions such as tetanus.
Because of the potential for side-effects, this drug is considered to be a high-risk medication for the elderly.
Unlike other carbamates such as meprobamate and its prodrug carisoprodol, methocarbamol has greatly reduced abuse potential. Studies comparing it to the benzodiazepine lorazepam and the antihistamine diphenhydramine, along with placebo, find that methocarbamol produces increased "liking" responses and some sedative-like effects, however, at higher doses dysphoria is reported. It is considered to have an abuse profile similar to, but weaker than, lorazepam.
Methocarbamol is the carbamate of guaifenesin, but does not produce guaifenesin as a metabolite, because the carbamate bond is not hydrolyzed metabolically; metabolism is by Phase I ring hydroxylation and O-demethylation, followed by Phase II conjugation. All the major metabolites are unhydrolyzed carbamates.
Methocarbamol without other ingredients is sold under the brand name Robaxin in the U.K., U.S. and Canada; it is marketed as Lumirelax in France, Ortoton in Germany and many other names worldwide. In combination with other active ingredients it is sold under other names: with acetaminophen (Paracetamol), under trade names Robaxacet and Tylenol Body Pain Night; with ibuprofen as Robax Platinum; with acetylsalicylic acid as Robaxisal in the U.S. and Canada. However, in Spain the tradename Robaxisal is used for the Paracetamol combination instead of Robaxacet. These combinations are also available from independent manufacturers under generic names.
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